With a steam turbine, the thermal energy of the steam is converted into mechanical work. The mechanical work is used to drive the condensation steam turbine. To this end, a shaft extending through the condensation steam turbine, the so-called rotor, is driven with the aid of turbine blades. The rotor is usually coupled to a generator for energy generation.
Rotor blades and guide vanes are provided to drive the rotor. The rotor blades are fastened to the rotor and rotate therewith. The guide vanes are fixedly arranged on the turbine housing. The guide vanes are molded and arranged here such that they provide for a favorable and efficient flow guide of the steam through the turbine and thus enable as effective a conversion of the thermal energy into mechanical work as possible. When converting thermal energy into mechanical work, both the temperature and also the pressure of the steam reduces. To achieve as high a degree of efficiency as possible, the condensation steam turbine is generally divided into different regions, a high pressure part and a low pressure part. In large power plants, the steam is currently reduced to approximately 50 mbar. In the case of turbines for industrial power plants which are configured for lower outputs, the steam in the low pressure part is reduced to a final pressure of 100 mbar. For efficiency reasons, in other words for as high a degree of efficiency as possible, attempts should basically be made to achieve as low a final pressure as possible. However, the necessary technical measures become ever more complicated and expensive as the final pressure reduces. The afore-cited current limits for the final pressure are essentially specified here by economical considerations. Newer developments aim to further reduce the afore-cited final pressures both in the case of large power plants and also in industrial power plants.
A problem associated with the low final pressures is the so-called water-droplet erosion, which results in considerable wear of the rotor blades. As a result of low pressures in the low pressure part of the condensation steam turbine, water is increasingly condensated out onto the stationary guide vanes. In this process, droplets form on the guide vane. The droplets are carried along here by the steam flow, from the rear edge of the guide vane, and then arrive at the rotating rotor blades with high energy, in particular at their leading edge. This effect produces a significant load on the rotor blades and in some unfavorable instances results in rapid and premature destruction of individual rotor blades.
The risk of water-droplet erosion is currently counteracted above all with mechanical means, in which the leading edges of the rotor blades are embodied in a particularly stable fashion. To this end, these are especially hardened or so-called stellite plates are applied thereto. Furthermore, with some guide vanes, the guide vane blades are heated with the aid of steam so that moisture deposition on the guide vanes can largely be eliminated as a cause of droplet erosion. For heating with steam, the guide vane is embodied with an internal cavity, through which cavity the steam is routed. The formation of guide vanes with a corresponding cavity and the supply of the steam to the cavity guide vanes however requires a significant constructive effort and is therefore expensive. Furthermore, steam heating encounters limiting factors in the case of small steam turbines, in particular in industrial steam turbines. As a result of the small guide vanes used there, these cannot be provided with a cavity for stability reasons.
The steam heating of rotor blades in the low pressure final stages is thus ruled out in the case of very small condensation steam turbines.
To also be able to heat guide vanes of smaller condensation steam turbines, EP 1 156 189 A1 proposes an electrical heating system. Here the vane is heated by way of an electrical heating resistor. The guide vane herewith comprises a central recess for receiving the heating resistor. The heating element is arranged here inside the guide vane. To guarantee good thermal transmission, the guide vane is embodied solidly from solid matter.
The disadvantage of these guide vanes is that a recess for the heating resistor has to be introduced into the solid matter. This recess negatively affects and/or reduces the stability and the mechanical properties of the guide vane. For stability reasons, the penetration depth of the borehole for receiving the heating resistor is restricted. As a result of the guide vanes embodied solidly from solid matter, high material costs also result and the weight of the guide vanes is relatively high.